This invention related to a thermal recovery process for recovering viscous oils from subterranean formations and, more particularly, to an in-situ combustion method for recovering such oils through producing wells which extend downwardly from the surface of the earth into the bottom of the oil-containing formation and then extend horizontally through the formation.
In-situ combustion is a commonly known method for recovering heavy viscous oils from subterranean formations. In this method, an oxygen-containing gas is injected into a reservoir through an injection well with ignition of oil within the adjacent reservoir initiated by means for establishing a combustion front. The reservoir is usually provided with one or more vertical production wells for the production of oil. As the flow of oxygen-containing gas to the reservoir is continued, the combustion front is moved from a vertical injection well toward the production wells. The heat generated by burning reduces the viscosity of the oil which is displaced ahead of the combustion front toward the production wells from which the oil is recovered. The combustion front, in displacing the mobile oil, uses the residual carbonaceous deposit as fuel. Examples of such in-situ combustion methods are found in U.S. Pat. Nos. 4,625,800 to Venkatesan; 4,566,536 to Holmes; and 4,474,237 and 4,454,916 to Shu, the teachings of which are incorporated herein by reference.
There are many subterranean formations containing heavy, i.e., viscous, oils. Such formations are known to exist in the major tar sand deposits of Alberta, Canada, and Venezuela, with lesser deposits elsewhere, for example, in California, Utah and Texas. The API gravity of the oils in these deposits typically ranges from 10.degree. to 6.degree. in the Athabasca sands in Canada to even lower values in the San Miguel sands in Texas, indicating that the oil is highly viscous in nature.
Various problems are associated with the in-situ combustion drive method. There is formed in front of the combustion front and relatively near the vertical injection well, a hot bank of hydrocarbons. The viscosity of this hot bank of hydrocarbons is much less than the viscosity of the hydrocarbons existing in the remainder of the reservoir and near the vertical production well. Thus, the capacity of the reservoir to flow hydrocarbons is much less near the production well than near the injection well. This results in a condition which is sometimes referred to as "fluid blocking". When this condition occurs, flow of the lower viscosity hot bank of hydrocarbons near the injection well is retarded by the slower rate of flow of the higher viscosity hydrocarbons near the production well. Under severe conditions where highly viscous fluids are present in the reservoir, the hydrocarbons near the production well may be relatively immobile and thus may, to a large extent, prevent the hot bank of hydrocarbons from flowing toward and into the production well. This results in a loss of efficiency and an excessive amount of the hydrocarbons may be burned in the reservoir.
It is therefore an object of the present invention to provide an improved method of heavy viscous oil recovery that will overcome such a "fluid blocking" as well as other problems by providing a horizontal production well in the lower part of the reservoir and establishing an in-situ combustion front in the upper part of the reservoir to allow gravity to assist the flow of the hot bank of hydrocarbons from the vertical injection well in the upper part of the reservoir to the horizontal production well in the lower part of the reservoir. Utilization of a horizontal production well will allow extended contact with the overlying reservoir, thereby facilitating gravity drainage and production of the heavy viscous oils from the overlying in-situ combustion zone.